There has conventionally been used a ceramic honeycomb structure as a diesel particulate filter (DPF) for trapping particulate matter, in particular, diesel particulate matter, in exhaust gas from an automobile.
Generally, such a honeycomb structure has porous partition walls separating and forming a plurality of cells extending between two end faces and an outer peripheral wall formed unitarily with the partition walls and has a structure where a plugging portion is formed in an end portion of each cell in such a manner that an end face and the other end face of the honeycomb structure have complementary checkerwise patterns (see, e.g., Patent Document 1).
FIG. 6 is a plan view partially showing an end face of a honeycomb structure (plugged honeycomb structure) having such plugging portions formed therein, FIG. 7 is a partial cross-sectional view showing an example of a d-d cross-sectional structure of FIG. 6, and FIG. 8 is a partial cross-sectional view showing another example of a d-d cross-sectional structure of FIG. 6. As shown in these figures, in a conventional plugged honeycomb structure 11, the plugging portion 7 formed in the outermost peripheral portion partial cells 5 each locating in the outermost peripheral portion of the honeycomb structure and having an incomplete cell cross section, a part of which is brought into contact with the outer peripheral wall 4, has a depth (length of plugging portions from an end face of the honeycomb structure) equivalent to that of the plugging portion 8 formed in the complete cells each having a complete cell cross section other than the outermost peripheral portion partial cells 5, or the plugging portion 7 formed in the outermost peripheral portion partial cells 5 has a depth larger than that of the plugging portion 8 formed in the complete cells 6 for the purpose of enhancing strength of an edge portion of the honeycomb structure to inhibit damages such as chipping.
When a plugged honeycomb structure is used for a DPF or the like, it is general that the plugged honeycomb structure is compression-fixed (subjected to canning) in a cylindrical tubular body in a state that a holding member of a ceramic fiber mat or the like is wrapped around the outer peripheral wall of the plugged honeycomb structure. As shown in FIG. 8, by the canning, canning surface pressure is applied on the plugged honeycomb structure 11. At this time, if the plugging portions 7 formed in the outermost peripheral portion partial cells 5 has a larger depth of the plugging than the plugging portions 8 formed in the complete cells 6, stress concentrates in a contact portion C between the inner side end portion of the plugging portion 8 formed in the adjacent cell 6 and the partition wall 3 due to action of bending moment generated by the load F in the vicinity of the inner side end portion of the plugging portion 7, and breakage is prone to be caused with the contact portion C being as a breakage starting point. Such a breakage can be caused also when the plugging portions 7 formed in the outermost peripheral portion partial cells 5 has a depth of plugging equivalent to that of the plugging portions 8 formed in the complete cells 6.
Patent Document 1: JP-A-2001-300922